(nytimes) -- “I think Baghdadi is alive,” Mr. Mattis told reporters at the Pentagon. “I’ll believe otherwise when we know we have killed him. We are going after him.”

The Russian military said last month that it had struck a meeting of ISIS leaders on May 28 near Raqqa, Syria, and possibly killed Mr. Baghdadi.

The Russian claim was given credence this month by a British-based monitoring organization, the Syrian Observatory for Human Rights, which said it had obtained information from senior ISIS commanders confirming that Mr. Baghdadi had been killed in Deir al-Zour Province. Raqqa Province neighbors Deir al-Zour in northeastern Syria.

The death of Mr. Baghdadi, who delivered a fiery sermon in Mosul, Iraq, after the militants captured the city in June 2014, would be a psychological blow to the ISIS, which is still using small groups of fighters to mount guerrilla attacks against Iraqi forces who have retaken the city.

Pentagon officials have said that Mr. Baghdadi, who takes stringent precautions to avoid detection, does not appear to be directing the ISIS' forces on a day-to-day basis. Mr. Mattis said he assumed that Mr. Baghdadi still “had a role to play” in the organization, but provided no details on what that role was.